The present invention relates to antennas for the transmission and reception of microwave energy. More particularly, the present invention relates to an improvement to a microwave antenna for reducing undesirable echo reflections to the feed system, which reflections are superposed, delayed, upon the desired transmitted and received microwave energy.
In the field of space communications, a microwave antenna is used to transmit and receive many communications channels. On such antenna is the Cassegrainian antenna, which has a large concave main reflector, a smaller convex subreflector placed forward of the main reflector and a feed system, often located centrally in an opening in the main reflector. Radiation from the feed is reflected from the subreflector to the main reflector and is transmitted from the antenna as a narrow microwave beam.
Unfortunately, some radiation transmitted from the feed is also reflected undesirably back to the feed from the subreflector. This undesirable reflection is called an echo, the echo corresponding with an impedance mismatch, in this case between the feed and the subreflector. The echo causes, for example, an objectionable intermodulation background noise component in frequency division multiplexed FM communications channels which sharply increases as the antenna size and number of channels is increased. See Bell Telephone Laboratories, Transmission Systems for Communications, 4th Ed., pp. 517-522, 1970.
Heretofore, undesirable echo reflections have been reduced by placing an essentially flat reflecting plate near the subreflector between the subreflector and the feed system to cancel some of the echo at the feed. When the plate reflects radiation to the feed which is equal in amplitude and 180.degree. out of phase at a given frequency with the echo at the feed location from the rest of the subreflector, complete echo cancellation at that frequency is obtained. As the number of communications channels is increased, however, the frequency range over which the sharply increased echo-caused noise can be acceptably cancelled by a flat plate decreases. Furthermore, some communications systems use distinct frequency ranges for simultaneous transmission and reception. Consequently, as the number of channels is increased to take full economic advantage of the antenna, the echo-caused noise in these frequency ranges rises above an acceptable level if a flat plate is employed. Accordingly, it is an object of the present invention to substantially cancel microwave echo reflections over a wide bandwidth in a microwave antenna.
It is another object of the present invention to substantially eliminate echo-caused channel noise from a Cassegrainian antenna accommodating a large number of communications channels.
It is another object of the present invention to substantially eliminate undesirable echo interference to simultaneously transmitted and received communications channels carried in distinct frequency ranges in a microwave antenna.
Attention is called to the copending application of E. A. Ohm entitled, "Antenna with Echo Cancelling Elements," Ser. No. 597,366, in which there is disclosed a dual frequency echo-cancelling structure having a gridded design. Also, improvements to the gridded design of E. A. Ohm are disclosed in the copending application entitled, "Antenna with Echo Cancelling Elements," Ser. No. 597,368, of G. W. Travis and myself. The present application, by contrast, reveals a dual frequency echo-cancelling structure having a distinctly different design which accomplishes objects including those stated above.